Mats Åbom

Error analysis of two‐microphone measurements in ducts with flow

In an earlier work [H. Boden and M. Abom, J. Acoust. Soc. Am. 79, 541–549 (1986)] the influence of errors on two‐microphone measurements in ducts without flow has been studied. The aim of this article is mainly to extend the earlier work to include the effects of mean flow and also of attenuation during the sound propagation. First, a short review of the various existing two‐microphone methods is made. The errors in the measured input data are then analyzed and special attention is paid to the effects of neglected attenuation, nonideal microphones, and flow noise. The influence of errors on the calculated quantities has been investigated and the conclusions from the earlier work have been extended to the case with flow. It is also shown that the neglect of attenuation between the microphones leads to a low‐frequency limit for the applicability of the two‐microphone method. Finally, a new technique for measuring the Mach number using a two‐microphone method is suggested.

Influence of errors on the two‐microphone method for measuring acoustic properties in ducts

Using the two‐microphone method, acoustic properties in ducts, as, for example, reflection coefficient and acoustic impedance, can be calculated from a transfer function measurement between two microphones. In this paper, a systematic investigation of the various measurement errors that can occur and their effect on the calculated quantities is made. The input data for the calculations are the measured transfer function, the microphone separation, and the distance between one microphone and the sample. First, errors in the estimate of the transfer function are treated. Conclusions concerning the most favorable measurement configuration to avoid these errors are drawn. Next, the length measurement errors are treated. Measurements were made to study the question of microphone interference. The influence of errors on the calculated quantities has been investigated by numerical simulation. From this, conclusions are drawn on the useful frequency range for a given microphone separation and on the magnitude of ...

Measurement of the scattering-matrix of acoustical two-ports

Abstract In this paper a method for measuring the scattering-matrix of an acoustical two-port is presented and tested. All earlier works on experimental characterisation of acoustical two-ports have focused on the measurement of the transfer-matrix. Since the scattering-matrix has not been tested before, it represents a new approach to the problem. It can be argued that this is the best approach since a scattering-matrix gives the most basic description of a wave interaction problem. However, when one type of matrix description of a two-port is known, all other types can be obtained from linear transformations. From this point of view the choice of description is not so important. A more important difference between this paper and earlier works is; the formulation of the measurement method. Here the basis for the method is a direct estimation of the state vectors. In the authors opinion this represents the most straight-forward formulation since it is based directly on the definition of a two-port matrix. One advantage with this formulation is that it can easily be extended to the general case of an N -port. Another difference between this paper and earlier works on the problem is the method to suppress disturbing noise. Here a reference signal, perfectly correlated with the acoustic field, is used. To suppress noise the transfer function between the reference signal and each state vector component is measured.

A New Type of Muffler Based on Microperforated Tubes

Microperforated plate (MPP) absorbers are perforated plates with holes typically in the submillimeter range and perforation ratios around 1%. The values are typical for applications in air at stand ...

Modal decomposition in ducts based on transfer function measurements between microphone pairs

Abstract A new in-duct modal decomposition technique is described. The basis for the technique is transfer function measurements between microphone pairs. The new technique is valid independently of the signal type present in the duct and it separates the modes into incident and reflected parts. The formalism is first developed for the general case of straight cylindrical ducts, and then the special case of ducts with circular cross-section is treated in detail. Measurement of acoustic power and reflection-matrices are also discussed. The measurement of reflection-matrices has many interesting applications, e.g., in studies of mufflers, and has to the authors knowledge not been discussed before. The influence of measurement errors on the modal decomposition is analyzed in detail for a special case. This case corresponds to circular ducts below the cut-off frequency for the first radial mode. For this case experimental tests of the new technique are also presented.

Derivation of Four-Pole Parameters Including Higher Order Mode Effects for Expansion Chamber Mufflers with Extended Inlet and Outlet

Abstract A derivation of the four-pole parameters including higher order mode effects, but with flow-acoustic interactions neglected, is presented for expansion chamber mufflers with extended inlet and outlet. The solution of the problem is based on a standard mode-matching technique. The solution is derived for the general case of straight cylindrical ducts with rigid walls. The end walls in the chamber are described as uniform and locally reacting surfaces. The validity of the solution is tested for the case of circular and concentric ducts.

Modeling perforates in mufflers using two-ports

One of the main sources of noise of a vehicle is the engine where its noise is usually damped by means of acoustic mufflers. A very common problem in the modeling of automotive mufflers is that of ...

Aero-Acoustics of Flow Duct Singularities at Low Mach Numbers

This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.

On the Use of Linear Acoustic Multiports to Predict Whistling in Confined Flows

The use of linear acoustic multiport models to analyse the existence of flow driven instabilities, that is - self-sustained oscillators or whistles - is addressed. By combining the scattering and r ...

Modeling and testing of after-treatment devices

Driven by emission regulations in the US and the EU exhaust systems on new diesel engines are equipped with both a catalytic converter (CC) and a diesel particulate filter (DPF). The CC and DPF are ...